1. Field of the Invention
This invention relates to a color cathode ray tube apparatus, and particularly to a color cathode ray tube apparatus which provides a high focusing characteristic and in which the deflection aberrations of electron-beams are reduced which are caused by the deflection magnetic field produced by a deflection device for use in the color cathode ray tube apparatus, and also relates to a deflection device for use therein.
2. Description of the Related Art
A color cathode ray tube apparatus of the shadow mask type comprises a panel section having a generally rectangular face plate and a skirt extending from a lateral edge of the face plate, a funnel section connected to the panel section, and a neck section continuously formed with the funnel section. The interior of the cathode ray tube is maintained in a vacuum state by the panel section, the funnel section and the neck section. In the neck section is housed an electron gun assembly which generates three electron beams (R), (G) and (B). At the outer lateral side of the portion of the apparatus between the funnel section and the neck section is disposed a deflection device for generating magnetic fields which deflect electron beams vertically and horizontally. A phosphor screen is formed on the inner face of the face plate of the panel. In the tube, a generally rectangular shadow mask is opposed to the face plate at a predetermined spacing. The shadow mask is made of a thin metal plate and provided with a lot of slit apertures.
The deflection device for use in the color cathode ray tube apparatus of shadow mask type has a horizontal deflection coil and a vertical deflection coil which produce magnetic fields for respectively horizontally and vertically deflecting the three electron beams (R), (G), (B) emitted from the electron gun assembly. After deflected by the horizontal and vertical deflection coils, the three electron beams (R), (G) and (B) are converged towards the corresponding slit. The electron beams (R), (G) and (B) converged at the vicinity of the slit are landed on the phosphor screen which has three kinds of phosphor stripes alternately arranged to each other. The three electron beams (R), (G) and (B) pass the slit and are incident on the phosphor screen, whereby the red light, green light and blue light are emitted from the phosphor stripes. In other words, the three beams are landed on the corresponding phosphor stripes which emit the red, green and blue light.
When the electron gun assembly is of inline type, the electron beam (G) which causes green light to be emitted is radiated from the electron gun so as to coincide with the tube axis. The electron beams (B) and (R) which cause blue light and red light to be emitted, respectively, are radiated, with the electron beam (G) disposed therebetween. The color cathode ray tube apparatus, which employs the characteristic of the inline type electron gun to produce specific non-uniform magnetic fields by means of deflection yokes, is a self-convergence type color cathode ray tube apparatus. With the color cathode ray tube of this type in which the three electron beams are radiated on the same horizontal plane, for example, a horizontal deflection magnetic field of mainly pincushion type and a vertical deflection magnetic field of mainly barrel type are applied. The impression of these magnetic fields enables the three electron beams radiated on the same horizontal plane to be converged on the phosphor screen.
However, with this inline type color cathode ray tube apparatus, the center beam (G) and the side beams (B) and (R) do not converge well each other to produce coma aberration on the peripheral portion of the screen. Japanese Patent Publications No. Sho 51-26208 and Sho 54-23208 describe an apparatus wherein an electron gun assembly is provided, for the correction of the coma aberration, with magnetic substance which changes a shape of the after-leakage magnetic field which is a part of the magnetic field generated by a deflection device. Japanese Utility Model Publication No. Sho 57-45748 discloses another apparatus in which an auxiliary coil is provided at the side of the electron gun of a deflection device and is adapted to render an electrical current to flow in synchronism with the deflection current flowing in a vertical deflection coil, thereby generating a strong pincushion type magnetic field.
With the color cathode ray tube apparatus in which the correction means for coma aberration is provided, however, the beam spots formed on a screen by the beams are distorted, because a non-uniform magnetic filed is employed as the deflection magnetic field. In other words, when a uniform magnetic field is impressed, the beam spots assume a substantially truly circular shape over the whole surface of the screen. However, referring to FIGS. 1, 2A and 2B, the beam spots 4 formed on the X-axis by the electron beams at the horizontal side end portions of the screen receive, by means of a pincushion type horizontal magnetic field, Lorentz's forces which press the electron beams above the X-axis downward and the electron beams below the X-axis upward, and the beam spots 4 are distorted to take an elliptical shape whose major axis extends horizontally. On the other hand, the beam spots 6 on the vertical axis (the Y-axis) receive, by means of a barrel type vertical deflection magnetic field 12, Lorentz's forces which press the electron beams at the right side rightwards and press the electron beams at the left side leftwards. The beam spots 4 are distorted to assume an elliptical shape whose major axis extends horizontally. The side beams receive Lorentz's forces whose magnitudes are different from each other at the right side and at the left side. In addition, the directions of the Lorentz's forces exerted on the electron beam (B) and the electron beam (R) are reversed to each other. Therefore, the beam spots formed by the two side beams at the vertical end sides at the vicinity of the vertical axis assume elliptical shapes whose major axes are crossed each other. As a result, the focusing characteristic at the peripheral portion of the screen is remarkably deteriorated due to the deformation and inclination of the beam spots caused by the horizontal and vertical deflection magnetic fields. The deterioration of the focusing characteristic is a serious cause that prevents the electron gun assembly from being highly efficient.
Accordingly, such compromise must be made for designing a color cathode ray tube apparatus that the focusing characteristic of the central portion of the screen is lowered to improve the focusing characteristic of the peripheral portion of the screen, thereby obtaining a uniform focusing characteristic over the whole area of the screen.
For the auxiliary coils as described in Japanese Utility Model Publication No. Sho 57-45748, the following defects are developed. Since an electrical current flows in synchronism with the deflection current flowing in the vertical deflection coil, a magnetic field for directing the beams vertically is produced, which is directed in the horizontal direction on the horizontal axis. The beams are excessively deflected by the magnetic filed at the side of the electron gun assembly of the deflection device and likely collide with the inner wall of the neck section. This causes on the shadow a so-called neck shadow which is a portion on which no beams arrive (that is, a non-luminous portion). The auxiliary coil comprises a magnetic substance and a coil wound therearound in which an electrical current flows. Thus, it is expensive as a correction element. In many cases, the impedances of the deflection devices are changed according to the requests of set makers which manufacture TV sets using color cathode ray tube apparatuses. When the impedance is changed, the current flowing in the deflection coil is also changed. Therefore it is necessary to modify the specification of the auxiliary coil according to the impedance of the deflection coil, in order to produce a proper effect of the auxiliary coil on the deflection device. This hinders the mass production of the color cathode ray tube apparatuses.